Projection welding is a known technique for joining two overlapping metal sheets. In conventional projection welding, a small projection is provided on one of the sheets and extends transversely to a side of the sheet so that the tip thereof contacts the other sheet. An electrode of a welding gun is pressed into contact with one of the sheets in alignment with the projection, and a combination of force and welding current is applied to the electrode to cause the projection to collapse and form a weld nugget which joins the two sheets together at the contact area defined by the projection.
FIG. 1 illustrates a known arrangement 10 for projection hem welding. This welding arrangement 10 is particularly desirable for projection welding of thin sheet-like metal components disposed in overlapping relation, typically light-gauge steel sheets which commonly have a thickness in the range of about 0.020 to about 0.050 inch. The thin sheets 11 and 12 have portions which directly overlap, and in a hem welding process one of the sheets 11 has a flange or hem part 13 which is bent to overlap another side of the other sheet 12. The sheet 12 is sandwiched between the sheet 11 and its hem part 13. Intermediate sheet 12 has a bead or projection 14 projecting transversely therefrom so that the tip of the projection contacts the adjacent surface of the hem part 13. A projection weld is created Directly between the sheet 12 and the hem part 13 at the region of the projection 14.
An adhesive 16 can be placed between the two sheets 11, 12 (FIGS. 2 and 3). The adhesive 16 is a heat curable adhesive that will further secure the two metal sheets together after it is activated by applying heat thereto, usually in an oven.
In the projection welding arrangement 10 as shown in FIG. 1, the overlapping sheets 11, 12 are typically positioned on a support or die 15, and a movable welding head assembly 17 is positioned adjacent the die 15 to effect the projection weld between the overlapped sheets. The welding head assembly 17 includes a hollow housing 18 having an electrode 19 movably supported therein and projecting outwardly for contact with the overlapped sheets in the region of the projection, and a spring 21 is confined within the housing 18 and acts against an inner face of the electrode 19 so as to urge the electrode outwardly into an extended position, in which position the electrode abuts an interior stop surface formed on the housing. The welding head assembly 17 is electrically connected to a stationary transformer 22. The power supply to the transformer 22, and hence the welding current supplied to the welding head assembly, is controlled by a suitable control unit 23. The transformer 22 has the primary coils 24 thereof connected to suitable electrical conductors 25 and 26 that supply electrical energy to the transformer 22. The secondary coils 27 of the transformer are in turn connected to electrical conductors 28 and 29, one of which is connected to the welding head assembly 17 and the other is connected to the workpieces 11, 12, such as being connected either to the die 15 or to an electrical contact gun 31. The contact gun 31 includes a support 32 such as a conventional double-acting pressure cylinder, normally an air cylinder, having a conventional electrode 33 movably supported thereon. The electrode 33 engages a part 34 of the intermediate sheet 12, which part 34 is shown as spaced from the overlapping portions of the sheets. The conductors 28 and 29 are typically constructed of a conventional flexible lamination so as to permit respective movement of the welding head assembly 17 and contact assembly 31 relative to transformer 22.
The welding head arrangement also includes a drive device 36 for effecting movement of the welding head assembly 17. The drive device conventionally comprises a pneumatic cylinder 37 having a housing 38, which is typically stationarily mounted spaced and separate from the welding head assembly 17. An extendible and contractible piston rod 39 extends from the housing 38 and couples the housing 18 of the welding head assembly 17 to the drive device 36. The piston rod 39 controls the movement of the assembly 17 into engagement with the sheets 11, 12 when a projection welding operation is carried out.
When projection welding as summarized above, the projection is typically stamped or embossed on the thin steel sheet by opposed dies which deform the sheet by forcing a portion thereof sidewardly so as to define a projection which projects sidewardly of the sheet generally in the direction of the force applied by the forming die. The projection typically comprises a geometric shape such as a truncated conical or partial spherical shape as it projects transversely from the sheet. This method of forming the projection and the resulting shape thereof necessarily results in the wall thickness of the projection being thinner than the thickness of the base sheet, and also typically results in the base wall of the projection (i.e. the portion of the wall where the projection joins to the base sheet) being disposed in a sloped or angled relationship relative to the plane of the sheet. These latter configurational features, however, have not detrimentally effected the ability of the projection to create proper quality welds between thin steel sheets since steel possess a high tensile strength and thus is able to withstand the significant compressive force applied thereto prior to reaching the actual melting or welding temperature. Premature collapse of the projection during projection welding of sheet steel has thus not presented a significant problem.
When projection welding an aluminum sheet, however, totally different melting temperature and tensile strength properties are exhibited by aluminum sheet in comparison to steel sheet, and accordingly repeatably and successfully effecting projection welding of aluminum sheets can not normally be achieved. More specifically, not only does aluminum possess a significantly lower strength than steel, but more significantly it has been observed that the yield strength of aluminum undergoes a significant decrease when aluminum is heated to a temperature between two and four hundred degrees F. (FIG. 4), and in fact this significant decrease in strength occurs over a very small temperature range which is still significantly below the melting or welding temperature for aluminum. The many prior attempts to projection weld thin aluminum sheets have hence mostly met with failure since the projections have exhibited premature collapse thereof at a temperature which is significantly below welding temperature. Thus, it has not been repeatably possible to properly maintain the requisite electrode pressure on the projection, nor has it been repeatably possible for the electrode to properly follow up the collapsing of the projection so as to maintain proper current-transmitting contact therewith. The proper contact and hence transfer of current to the projection, and the proper concentration of the current through the small electric contact area defined by the projection, have thus not been dependably and repeatably achievable, and accordingly proper weld nuggets have not typically been achievable when attempting to projection weld aluminum sheets.
Accordingly, it is an object of this invention to provide improvements for projection welding of thin metal sheets and particularly improvements applicable for permitting successful projection welding in situations where the thin metal sheet having the projection formed thereon is constructed of aluminum.
More specifically, it is an object of the invention to provide an improved projection which is monolithically associated with a thin aluminum sheet for permitting successful performance of a projection welding operation, which projection due to its improved shape, size and forming process provides significantly increased strength so as to withstand the electrode pressure imposed thereon without experiencing premature collapse, thereby maintaining both proper interface pressure, contact area, and current density between the projection and the adjacent metal body until reaching the temperature at which the projection rapidly collapses and permits creation of a metallurgical bond between the overlapped metal sheets and body.
In the present invention, the projection is formed generally as a hollow upright cylindrical wall which is cantilevered perpendicularly away from one side of the aluminum sheet. The hollow upright wall has a wall thickness which approaches or is substantially equal to the thickness of the sheet, and also has significant height so as to define a column having significant strength against compression. The column where it joins the sheet is also backed by a full thickness of the sheet. The interior of the column opens downwardly from the free end thereof so as to terminate at a bottom wall which, in a preferred embodiment, is substantially flush with the upper surface of the sheet but is of significantly reduced thickness in comparison to the sheet thickness. A forming recess opens inwardly from the opposite side of the sheet in coaxial alignment with the projection and terminates at the thin bottom wall. The forming recess typically is limited by an outer side wall which does not extend radially beyond the radial extent of the inner wall of the hollow upright cylindrical wall.
The projection of this invention is preferably formed by a coining operation whereby the sheet material is squeezed sidewardly during forming of the bottom recess and is then forced to flow outwardly (i.e. perpendicularly) so as to permit creation of the hollow upright cylindrical wall. The material forming the upright thus undergoes significant cold working which not only effects creation of the thick upright wall so as to provide significant column strength, but also effects significant cold working of the material so as to further increase the strength thereof.
The present invention also includes an improved welding gun which, in conjunction with the improved projection, permits performance of successful projection welding of thin metal sheets, particularly aluminum sheets. The welding gun employs a spring-urged electrode which possesses minimal mass and inertia so as to readily respond to the collapse of the upright projection during the welding operation, thereby maintaining proper contact pressure and concentration of the welding current to thus result in a desired weld nugget or joint.
Other objects and purposes of the invention will be apparent to persons familiar with technology of this general type upon reading the following specification and inspecting the accompanying drawings.